I'd go to the doctor and see if you can get a blood test or ultrasound if you're pretty sure you're pregnant. I've never heard of breastfeeding interfering with tests, but I've never researched it either.
I think I was in the second half of the first trimester sometime when I started getting round ligament pain if I sneezed or coughed. Sometime around 9 or 10 weeks, maybe? I'm not really positive, though.